1. Second Law of Thermodynamics Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature. The entropy of an isolated macroscopic system never decreases, or (equivalently) that perpetual motion machines are impossible.

2. Second Law of Thermodynamics Identifies the direction of a process. (e.g.: Heat can only spontaneously transfer from a hot object to a cold object, not vice versa) Used to determine the “Quality” of energy. (e.g.: A high-temperature energy source has a higher quality since it is easier to extract energy from it to deliver useable work.) Used to exclude the possibility of constructing 100% efficient heat engine and perpetual-motion machines Used to introduce concepts of reversible processes and irreversibilities. Determines the theoretical performance limits of engineering systems. (e.g.: A Carnot engine is theoretically the most efficient heat engine; its performance can be used as a standard for other practical engines)

3. Second Law (cont) A process can not happen unless it satisfies both the first and second laws of thermodynamics. The first law characterizes the “quantity” of energy. The second law defines the “quality”. Define a “Heat Engine”: A device that converts heat into work while operating in a cycle.Heat Engine Heat engine QHQH QLQL THTH TLTL W net  Q-W net =  U (since  U=0 for a cycle)  W net =Q H -Q L Thermal efficiency (Carnot efficiency),  th is defined as:  th =W net /Q H =(Q H -Q L )/Q H =1-(Q L /Q H )

4. Kevin-Planck Statement The Kelvin-Planck Statement is another expression of the second law of thermodynamics. It states that: It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce net work. Heat engine QHQH THTH W net A heat engine has to reject some energy into a lower temperature sink in order to complete the cycle. T H >T L in order to operate the engine. Therefore, the higher the temperature, T H, the higher the quality of the energy source and more work is produced.

5. Reversible Processes and Irreversibilities A reversible process is one that can be executed in the reverse direction with no net change in the system or the surroundings. At the end of a forwards and backwards reversible process, both system and the surroundings are returned to their initial states. No real processes are reversible. However, reversible processes are theoretically the most efficient processes. All real processes are irreversible due to irreversibilities. Hence, real processes are less efficient than reversible processes. Common Sources of Irreversibility: Friction Sudden Expansion and compression Heat Transfer between bodies with a finite temperature difference.

6. Entropy Systems will tend to progress towards a state of entropy (lower levels of order).

7. Summary While the first law of thermodynamics is considered “written in stone”, the second law is about the probability of an occurrence. Sometimes order does appear from disorder. However, unless work is added to the system, the likelihood of that happening is small.